Recent experiments indicate that: (1) Ingestion of increased amounts of the trace mineral selenium during the period of tooth development is associated with increases in dental caries in man and experimental animals, (2) Selenium taken up into enamel and dentine is associated with the proteins of these dental tissues. (3) Selenium does not prevent the postdevelopmental uptake of fluoride onto rat molar enamel. Utilizing the continuously growing rat incisor, we will study fluoride-selenium interactions in dental enamel at various stages of tooth development in rats receiving increased amounts of selenium and fluoride. Using the rat molar caries model, we also plan to study the effect of selenium deficiency on the development of dental caries and the cariostatic properties of fluoride.